Precious metal composition and artifacts made therefrom

ABSTRACT

A decorative artifact made of a precious metal composition containing 3-9% by weight aluminum, 1-6% by weight titanium, and the balance being platinum, in which the hardness of the composition is at least 600 on the Vickers scale.

FIELD OF THE INVENTION

This invention relates to precious metal compositions having uniquephysical properties and to artifacts made therefrom. More particularly,but not exclusively, the invention relates to precious metal artifactswhich are hard enough to be faceted and/or polished using techniquescommonly used on gemstones; and, have a composition which may beselected to ensure that the precious metal conforms to requiredhallmarking regulations, particularly insofar as platinum and goldmaterials are concerned.

The artifacts may assume the form of faceted or polished cabochon“gemstones” or wrist- or other watch casings, facings and dials andstraps as well as non-faceted watch casings, facings, dials and strapsor any other decorative artifacts which do not require materialworkability for their production. Such artifacts will be generallytermed “decorative artifacts” herein. It is to be understood that theterm cabochon means an artifact having a non-faceted domed surface ofwidely variable shape in plan view.

BACKGROUND TO THE INVENTION

There are no naturally occurring, silver-coloured, metallic gemmaterials with a precious metal, in particular platinum contentsufficiently high to provide a high intrinsic value, and in particular,to be hallmarkable, and with a Mohs hardness greater than 5.

Pure platinum and platinum alloys with Vickers hardnesses between 50 and300 are too soft to be cut, optionally faceted, and polished to formdurable “gemstones”. Synthetic and natural gemstones used in theproduction of watch casings, facings or dials generally have Moh'shardness greater than 5 and the commonly used sapphire (corundum) has aMohs hardness of 9. This ensures good polish and suitable wearproperties. Natural metallic gemstones such as haematite have Mohshardnesses of 6.5 to 7.5.

Pure platinum and existing platinum alloys cannot approach thesehardness values. Even existing hard platinum alloys are not sufficientlyscratch resistant to prevent scratches tom dulling the polished surface.

Other methods of increasing the hardness of platinum involve surfacetreatments. Hardness values of up to 700 Vickers (Mohs hardness 6) havebeen reported, sufficiently hard to polish to a highly reflectivesurface finish. However, only a thin surface layer up to 200 micrometresthick is affected and polishing, finishing, wear etc could result incompromising the integrity of the layer. Furthermore, these surfacelayers are often brittle, prohibiting subsequent mechanical work.

Plating finished platinum articles can also give a highly reflectivesurface finish, but has the disadvantage that it can discolour, wear offand needs expertise if it is to be done well.

Coloured platinum intermetallic compounds have been described in theprior art in South African Patent No. 90/7777. This product,unfortunately, suffers from two disadvantages, namely:

(i) it does not produce a silver coloured material, and,

(ii) the platinum content is too low to conform to most, if not all,hallmarking requirements.

In the case of platinum, relatively high platinum contents are requiredfor hallmarking purposes such as, for example, in the case of Japan, notless than 85 weight percent; and in the case of the USA, not less than95 weight percent platinum. In the USA an alternative is that there be50% platinum by weight with the balance of the 95%, namely 45%,comprising other platinum group metals. In Europe, the requirement iscommonly 95 weight percent platinum.

OBJECT OF THE INVENTION

It is an object of this invention to provide hard, durable, alloys andcompounds based on precious metals having a sufficiently high content ofprecious metal to enable it to conform to hallmarking standards and alsowhich is capable of being faceted to provide metallic “gemstones”.

It is to be understood that in this specification the term “alloyingmetal” will be employed whether or not the alloying metal forms a solidsolution or immiscible composite with the dominant precious metal, or itforms an intermetailic compound with such precious metal, or a two-phasemixture of the two.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a decorativeartifact made of a precious metal composition containing a minimum of55% by weight platinum group metal with a total precious metal contentof at least 80% by weight, together with one or more alloying elements,and wherein the proportions of the various elements are chosen such thatthe precious metal composition includes intermetallic compounds andalways has a Vickers hardness of at least 600.

The fact that the composition always has a Vickers hardness of at least600 means that it cannot be softened to a lesser hardness by any processor heat treatment. Further features of the invention provide for theprecious metal composition to conform to a required standard, inparticular a hallmarking standard; for the precious metal to bepredominantly platinum; for the precious metal, and more particularlythe platinum group metal content to be greater than 85%, and morepreferably greater than 90% by weight; for the alloying metals, in thecase of platinum as the platinum group metal, to be titanium andaluminium in amounts from about 1 to 6% in the case of titanium, and3-9% in the case of aluminium; and for the Vickers hardness to begreater than 650 and preferably greater than 700.

Platinum material which contains at least 85 weight percent platinumwith the balance being aluminium may contain any one or more of theintermetallic phases Pt₃Al, Pt₅Al₃, PtAl or Pt₂Al; or any of thesestructures where titanium has partially substituted for platinum oraluminium.

The high hardness of these compositions implies that they can take ahigh polish or finish and develop colour.

EXPERIMENTAL TESTS

In order to test the invention, artifacts were produced from fourdifferent precious metal compositions based on platinum by meltingappropriate compositions of platinum, aluminium and titanium in anelectric arc furnace under an inert atmosphere, in particular argon, toproduce button-shaped melts. The samples were cooled, turned over andre-melted twice to ensure homogeneity.

The buttons contained intermetallic compounds and were cut and polishedusing standard faceting and polishing techniques to provide “gemstones”.

The various compositions produced were as follows:

Weight % Pt Ti Al Vickers hardness 10 kg load 91.5 1 7.5 713 ± 24 91.5 26.5 724 ± 15 90.0 3.3 6.7 897 ± 24 92.0 1.5 6.5 679 ± 44

It is to be noted that a Vickers hardness of around 700-800 isequivalent to 6 to 6,5 Mohs.

It will be seen that numerous compositions are possible within the scopeof this invention provided they conform to the required hardness of 650to 800 specified above.

What is claimed is:
 1. A precious metal composition comprising from 1%to 6% by weight titanium, and 3% to 9% by weight aluminum, the balancebeing platinum and unavoidable impurities, wherein the proportions ofthe various elements are chosen such that the composition includesintermetallic compounds and always has a Vickers hardness of at least600.
 2. The precious metal composition of claim 1, wherein thecomposition includes at least one intermetallic phase selected from thegroup consisting of Pt₃Al, Pt₅Al₃, PtAl, and Pt₂Al, wherein the platinumor aluminum may optionally be partially replaced with titanium.
 3. Adecorative artifact manufactured from the composition of claim 1 orclaim
 2. 4. A precious metal composition comprising from 1% to 6% byweight titanium, and 3% to 9% by weight aluminum, the balance beingessentially platinum, wherein the proportions of the various elementsare chosen such that the composition includes intermetallic compoundsand always has a Vickers hardness of at least 600.